The disclosed invention generally relates to liquid crystal light valve projectors, and is particularly directed to a liquid crystal light valve projector having respective cathode ray tube light valve assemblies with coplanar optical axes and which can be packaged in a small volume.
The development of liquid crystal light valve (LCLV) technology has resulted in the development of large screen projectors which utilize one or more LCLV's to modulate the light being projected. The LCLV's are selectively modulated by respective appropriate apparatus such as cathode ray tubes (CRT).
Color liquid crystal light valve projectors typically include color separating apparatus such as prisms or beamsplitters for separating white light into the three primary color bands; namely, red, green, and blue light components. The red, green, and blue components are individually modulated by respective light valves for projection. Examples of color LCLV projectors are set forth in U.S. Pat. No. 4,425,028, issued to R. J. Gagnon et al. on Jan. 10, 1984, and assigned to the assignee of this application; and in U.S. Pat. No. 4,461,542, issued to R. J. Gagnon on July 24, 1984, and also assigned to the assignee of the subject application.
As noted above, various color separating apparatus have been and are utilized to separate the primary colors for respective individual modulation. For example, dichroic mirrors have been utilized, but they tend to exhibit astigmatism and polarization sensitivity. The article "Colour Separation in Colour-Television Cameras," DeLang et al., Philips Tech. Rev., Vol. 24, No. 9, pp. 263-298, 1963, discloses a three color separating prism assembly for avoiding the problems associated with dichroic mirrors, and which includes dichroic mirrors immersed in separate solid glass prisms or wedges. However, in view of the heat generated in a projector, such a prism assembly would probably suffer from stress birefringence if used in a projector, and was primarily used in television cameras where heat generation is substantially less significant. Accordingly, known liquid crystal light projectors could not utilize the advantages of a color separating prism assembly having glass prisms or wedges, and instead utilized other color separation structures which do not provide the same performance.
Some known color separating apparatus include dichroic mirrors which intersect each other. In addition to the care required in manufacture, possible image degradation resulting from the intersecting edges may be an important consideration.
A characteristic of some known liquid crystal light valve projectors is that the respective color channels include unequal path lengths due to the optical structure of the color separation apparatus. As a result, image quality is degraded.
Some other known liquid crystal light valve projectors utilize both axes of polarization, and as a result are characterized by reduced contrast. Further, the use of both axes of polarization generally requires placement of the CRT light valve assemblies in different planes. The use of both axes of polarization may be an important consideration in some display systems including those known as "in-line infinity" display systems or "pancake window" display systems. In-line infinity display systems, which contain polarizing elements, provide better transmission efficiency when illuminated with polarized light having a single axis of polarization. Illumination with two axis polarized light or unpolarized light would result in a substantial amount of light being removed by polarizing elements. Examples of in-line infinity display systems are referenced in a Farrand Optical Co., Inc. brochure M-129B, November 1974.